Efficiency of an antenna is usually defined as the ratio between the power the antenna radiates and the power put into the antenna by a coupled transmitter. Obviously, a high efficiency is usually desirable in an antenna.
The physical size of an antenna, normalized to its operating wavelength, is usually referred in the art as the “electrical size” of the antenna, so a “small antenna” usually means an Electrically Small Antenna (ESA). Clearly, small antennas are desirable, particularly in mobile device.
In addition, embedding an antenna in a substrate obtaining a dielectric constant larger than 1 (which is the dielectric constant in free space, and approximately the dielectric constant in air), can reduce the antenna size, for a given efficiency, by the square root of the substrate dielectric constant. Yet, in the present document, a dielectric constant of 1 is assumed, unless specified otherwise.
Ideally, a small and efficient antenna should be designed for most wireless devices, however, a well known rule of thumb trades off between these two parameters, limiting the miniaturization of the electrical size of an antenna, for a given efficiency. Basically, this rule dictates that at least one of the antenna dimensions should be not less than ¼λ, where λ (lambda) is the transmission (or reception) wavelength, to achieve efficient radiation.
This rule is well reflected in one dimensional antennas (“whip” or “rod” shaped) such as λ/2 dipoles and λ/4 monopoles (the latter normally placed over a ground plane). For example: a λ/2 dipole of 37 cm for 406 MHz (for emergency radio beacons); a λ/4 monopole of 1 meter for 156.8 MHz (channel 16, for distress safety and calling in the marine VHF band).
Also two dimensional antennas (planar antennas) such as patch or microstrip, reflect this rule. For example: a λ/2 patch antenna of 6×6 cm for 2.4 GHz (WiFi); ˜3×3 cm for 1.575 MHz (GPS-L1), on a high dielectric substrate.
Three dimensional antennas also reflect this rule. For example, efficient axial mode helical antennas typically obtain a coil diameter of λ/3.
Clearly, smaller than λ/4 antennas can be tuned for a specific frequency, yet this usually degrades the antenna efficiency. Thus, an efficient antenna for a relatively low frequency (i.e. long wavelength) is not easily achieved in small dimensions. Practically, in mobile or portable communication devices, this limitation is particularly relevant to UHF, VHF and lower frequency bands.
Over the years, more complex shapes of antennas, many of them three dimensional, were been studied. Some fundamental works were been published by Wheeler [H. A. Wheeler, “Fundamental Limits of Small Antennas,” Proceedings of The I.R.E. (IEEE), December 1947, pg. 1479-1484], Chu (Chu, L. J, “Physical Limitation of Omni-Directional Antennas”, Journal of Applied Physics, Vol. 19, p. 1163-1175, December 1948) and others. Based on these works, theoretical arguments predict that the minimal size for practical antennas will require a volume of half a sphere with a radius r, where kr=0.3 (k=2π/λ). For example, at 406 MHz this means a radius r of ˜4 cm.
Still, not surprisingly, some communication devices employ antennas configured for two positions: a stowed position, where the antenna is usually retracted or coiled or folded in a compact way, and an operational position, typically less often used, where the antenna is extended to a larger size, in order to improve its gain.
The size issue becomes much more challenging when two or more antennas are required to be placed in the same communication device. Then, each of the antennas has its size reduction limitations, as discussed above, but also, the electromagnetic coupling between the antennas is important, as one antenna might load and reduce the performance of another, if collocated too close. For the purpose of simplicity, the discussion hereby will be limited to two antennas collocated in the same device, however as a skilled person may appreciate, this may be broadened to more than two antennas.
A typical example for a small wireless device employing two antennas is a personal device obtaining one antenna to receive Global Navigation Satellite System (GNSS, such as GPS) signals, and another antenna to communicate over VHF/UHF bands, in order to report its location. It is estimated that more than a hundred millions of mobile (cellular) devices presently comprise a GPS receiver for navigation or Location Based Services (LBS), in additional of course to a UHF (e.g. 900, 1800 or 1900 MHz) cellular transceiver.
Devices that confront the antenna size issue more seriously, due to a lower operation frequency, are distress radio beacons operating on 406 MHz. Such a device, also known as PLB (Personal Locator Beacon), is designed to be carried by a person, and operate in case of emergency. When activated, the PLB repeatedly transmits short data bursts at 406 MHz, indicating the beacon Identification Data (ID) and its location. Since such PLBs are to be detected by satellites, including satellites orbiting 35,000 Kilometers above the earth, and since the PLB output power is only 5 watts to enable a reasonable operation time on batteries, an efficient antenna is required. Thus, many PLBs use a ½ lambda dipole antenna, about 37 centimeters long. Such are PLBs which are compatible with the Cospas-Sarsat Search and Rescue (SAR) satellite system. Though the present invention is not limited to this specific system, Cospas-Sarsat is a good example to clarify the present art, its limitations and the present invention, so it is specifically enlightened here.
Cospas-Sarsat is a satellite communications system to assist SAR of people in distress, all over the world and at anytime. The system was launched in 1982 by the USA, Canada, France and the Soviet Union (Russia) and since then, it has been used for thousands of SAR events and has been instrumental in the rescue of over 24,000 lives worldwide. The goal of the system is to detect and locate signals from distress radio beacons and forward the data to ground stations, in order to support all organizations in the world with responsibility for SAR operations, whether at sea, in the air or on land. The system uses spacecraft —Low Earth Orbit (LEO) and Geostationary (GEO) satellites; and in the future also Medium Earth Orbit (MEO) satellites; Cospas-Sarsat radio beacons transmit in the 406 MHz band (and 121.5 MHz until 2009). The position of the beacon is determined either by the Doppler shift of the received beacon signal or by position data modulated on the signal, provided by a GNSS receiver integrated in the radio beacon.
A detailed description of the Cospas-Sarsat System is provided in the document “Introduction to the Cospas-Sarsat System, C/S G.003”, accessed through—http://cospas-sarsat.org/Documents/gDocs.htm
All Cospas-Sarsat beacons are subject to the same basic RF specifications, yet may employ a different mechanical structure and different activation method, possibly also slight differences in the data modulated on the signal, usually adopted to different applications, and named accordingly: a) Emergency Position Indicating Radio Beacon (EPIRB) for marine use; b) Emergency Locator Transmitter (ELT) for aviation use; and c) Personal Locator Beacon (PLB) for personal and/or terrestrial use. For the purpose of the present invention, the name “PLB” is mainly used, however it refers to any type of radio location beacon (not necessarily related to “persons”).
State of the art PLBs are in the size of a PDA (i.e. a palm top computer) or a “walkie-talkie”, designed to be hand held and require extending the antenna, about 37 centimeters long, when operating the device. Clearly, holding an operating PLB, with its antenna extended and kept substantially vertically to enable good transmission conditions, might well disturb a person in distress. Normally, a person in distress has further tasks to do beyond holding the beacon in a certain position; such a person might need his hands free to swim or run or row or climb, in addition to keeping the PLB nearby.
A possible solution for a PLB attached to a person in distress, enabling hands free operation, is a wrist-worn PLB. Yet, fitting a proper antenna to a wrist-worn PLB is not easy, due to the electrical size of a 406 MHz antenna and considering the interfering of the human body to RF radiation.
One solution for an antenna for such a wearable PLB was already considered by the applicant, who proposed a “Wrist Worn Communication Device coupled with Antenna Extendable by the Arm”, U.S. patent application Ser. No. 11/938,311, filed on 12 Nov. 2007.
Another type of devices that require small yet efficient antennas is devices for man overboard (MOB), i.e. a device worn by a person enabling his rescue upon accidently falling overboard a vessel in the open sea (or ocean, or lake, or river, etc.). Fast detection and location of such accidents is crucial, since survival time in water is limited, typically less than 2 days at ˜20° C. and less than 6 hours at ˜10° C.
Basically, an MOB device may comprise an RF transmitter, possibly also a GNSS receiver, in a small housing preferably worn on the wrist or the arm. On the ship, a compatible receiver is standing-by to detect signals from such a worn device, indicating the presence of the MOB. Clearly, a worn device should be small enough to enable continuously wearing it onboard. Furthermore, the device should obtain a transmission range long enough to be detected by its vessel or by a SAR team, even at low communications and low visibility conditions. Present devices usually operate on unlicensed bands, thus restricted to a low transmission power (typically less than 100 mw), so consequently obtain a poor transmission range. Therefore, an efficient antenna, yet in a small and user friendly device is probably desirable for any such MOB device.
U.S. Pat. No. 7,424,316 to Boyle discloses a Body-worn personal communications apparatus. According to Boyle, the antenna is mounted in the device such that one dimension of said antenna is aligned with the height of the device casing, and is designed so as to not require manipulation by a user. Boyle also discloses that this antenna is a helical antenna.
Boyle discloses a helical GSM antenna at 900 MHz, of 5 mm diameter and 10 mm height. Yet, an efficient antenna for a lower frequency would probably be bigger. Furthermore, Boyle refers only to a single antenna for a body-worn device, however when an additional antenna is required, such as for GPS, volume and area limitations become stricter.
Helical antennas provide significant advantages relevant to body worn communication devices, as well as for satellite communications, specifically due to the antenna circular polarization. However placing two helical antennas in a small communication device, particularly dealing with lower than GSM frequencies, would hardly enable a compact design and electromagnetic decoupling between the antennas.
U.S. Pat. No. 5,852,401 to Kita (Casio), discloses a wristwatch type distress message sending device worn by the user. The device comprises a GPS receiver, at least two sensors, distress signal generating means and a radio for sending the distress signal. The radio sending antenna is preferably a helical antenna contained in the watch body or alternatively, a helical whip antenna extending from the watch body. The GPS antenna, according to Kita, is planar and placed in the wristband of the watch. Kita fails to teach how such a helical whip antenna is installed in the watch, or extend from the watch. Furthermore, Kita fails to teach if this whip antenna is a dipole or monopole, and also fails to teach a proper ground plane, in case of a monopole.
U.S. Pat. No. 4,673,936 to Kotoh (Mitsubishi), discloses a small-size transmitting apparatus for search and rescue operation (SARTR) adapted to be worn by a user for emitting a microwave rescue signal upon a marine accident involving the user. Kotoh discloses a device operating on microwaves, typically on the 9 GHz band, where ½ lambda is less than 2 cm; however, for VHF/UHF, where antenna size is significantly bigger, this invention would require a different antenna design.
U.S. Pat. No. 5,559,760 to Schneider (Breitling), discloses a wristwatch comprising, in addition to a device for measuring and displaying the time, a high-frequency transmitter and an extensible antenna in the form of two wires wound up in two different housings of the watch before use; the antenna being unfurled by pulling on plugs fastened to each end of the antennas. The dipole antenna of this device is configured that once been extended, does not flex but remains straight. This method might be problematic since such a whip dipole antenna is quite long for VHF/UHF bands.
U.S. Pat. No. 6,987,708 to Megner et al., discloses an emergency call transmitter adapted to be attached in a threaded recess of a wristwatch housing, the call transmitter comprising a transmitter housing carrying an emergency signal-emitting mechanism, and an extractible antenna, wound up at first in said housing. This device also requires a quite inconvenient manipulation of the antenna to be placed in a proper transmission position, and as the antenna is extended, it would probably disturb the operator from freely move his hands.
The paper “Low Profile Integrated GPS and Cellular Antenna”, by Nathan P. Cummings, is a thesis submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering. See—http://scholar.lib.vt.edu/theseslavailable/etd-11132001-145613/unrestricted/etd.pdf. This paper presents a design for a compact, low profile antenna unit, operating at both the cellular band of 824-894 MHz and the GPS L1 frequency of 1575 MHz. The combined antenna unit is less than 10 cm in diameter and less than 5 cm in height. The two collocated antennas comprised in this unit are: a) a Planar Inverted F Antenna (PIFA), for cellular communications; and b) a patch antenna, for GPS, located on top of the PIFA. Scaling the PIFA dimensions for lower frequencies, such as 406 MHz instead of 850 MHz, would make it twice as big.
U.S. Pat. No. 5,909,197 to Heinemann et al. discloses a Deployable helical antenna stowage in a compact retracted configuration. Heinemann discloses a compressible and deployable antenna comprised of a top and a bottom plate, and a deployable structure fitted between the plates which can forcibly separate the plates and extend a helical antenna placed between the plates. Yet, Heinemann teaches that the force means to extend the antenna comprise spring-like members in radial slots in the inner side of said plates, whose bias is toward the axis.
U.S. Pat. No. 5,721,558 to Holemans also discloses a Deployable helical antenna. This antenna, mainly designed for spacecraft, as Heinemann's antenna, is configured to collapse to a stowed position, and deploy by means of springs and guiding plates, however not solely by own antenna spring force.
U.S. Pat. No. 5,216,436 to Hall et al. disclose a Collapsible, low visibility, broadband tapered helix monopole antenna. The antenna is comprised of a conductor formed as a tapered helix, which extends along an axis from a first helix diameter portion to a second helix diameter portion larger than said first helix diameter portion, and a plurality of electrically conductive attachment elements, which attach different helix diameter portions to one another; when allowed to expand toward its deployed configuration, these elements limit the expansion of the antenna.
The present art methods described above have not yet provided a satisfactory solution to the problem of a portable communication device, operating on a relatively low frequency, obtaining a compact size yet efficient antenna.
Furthermore, the present art methods described above have not yet provided satisfactory solutions to the problem of a small communication device with two efficient antennas, still compact enough to be carried routinely by a person.
It is the object of the present invention to provide a device and method for a small communication device coupled with one or two electrically efficient antennas, configured to be easily carried by a user, operated in a friendly manner and limiting as less as possible the user while the device is operating.
It is another object of the present invention to provide a device and method for collocating two antennas in a small volume, compared to the transmission wavelength, yet achieving a substantial electromagnetic decoupling between the antennas.
It is also an object of the present invention to provide a device and method for a communication device, comprising a GNSS (GPS) receiver with a matching antenna, and a VHF/UHF transmitter with a matching antenna, configured to be wrist-worn.
It is yet an object of the present invention to provide a device and method for a device comprising an antenna coupled to a transmitter, configured to transmit automatically from time to time, wherein the user is been indicated that the transmitter is about to be activated or is actually transmitting, and possibly also indicating a transmission acknowledgement, so the user could accordingly manipulate the device and antenna orientation in advance to increase the communication success probability.
It is yet another object of the present invention to provide a device and method for a wrist-worn emergency radio beacon, also known as a PLB, compatible with satellite systems such as COSPAS-SARSAT, coupled with an efficient RF antenna and a GNSS antenna, yet enabling as possible user activities such as swimming, skiing, climbing, rowing, and running.
It is still an object of the present invention to provide a device and method for a distress radio beacon, wearable by a person onboard a vessel and used for MOB Search and Rescue.
Other objects and advantages of the invention will become apparent as the description proceeds.